This invention relates to an apparatus for protecting a transformer and more particularly to a transformer differential relay for disconnecting the transformer from a power source supplying the transformer upon the detection of an internal fault in the transformer.
Differential relays are applied as primary protection for power transformers rated 10 MVA or larger to permit the rapid isolation of the power transformer from a power source to limit damage to the transformer and to minimize the threat to system stability. In the prior art transformer differential relay current transformers are connected to the primary and secondary windings of the protected power transformer to monitor the magnitude of the currents flowing into and out of the transformer. An internal fault in the power transformer, is indicated when the current flow into the power transformer exceeds the current flow out. This difference is termed the differential or operating current. A current differential relay compares the differential current with a predetermined value, such as the larger of the primary and secondary winding currents, and when the differential current exceeds a percentage of the predetermined value the relay disconnects the power transformer from the power source.
Power transformers, by their very nature, have different values of rated current for their primary, secondary, and tertiary windings. Even with careful selection of current transformer ratios there may be considerable mismatch, between the primary and secondary current provided to the relay. If the relay does not compensate for this mismatch, then it may appear as a differential current in the relay. To obtain matching of unequal current transformer secondary currents, taps are provided to balance the currents applied to the measuring circuits of the relay. The traditional tap values are 2.9, 3.2, 3.5, 3.8, 4.6, 5.0, and 8.9. However, these traditional tap values have an effect on the accuracy of the relay.
Another problem unique to transformer differential relays is the 30.degree. phase shift that occurs when the power transformer is connected wye/delta. If the 30.degree. phase shift between the currents monitored in the primary winding and the secondary winding is not balanced then it may appear as a differential current to the relay and the relay will fail to operate properly. Traditionally, compensation for this problem has been accomplished outside the differential relay either by the current transformer connections or by using auxiliary current transformers. The most common method is to connect the current transformers to complement the power transformer connection. If the power transformer's primary is wye connected, its current transformers are connected delta; and if the power transformer's secondary is delta connected, its current transformers are connected wye. The auxiliary current transformer method utilizes additional current transformers to equalize the current to the relay and to compensate for the 30.degree. phase shift associated with wye/delta connections.
Transformer differential relays are also subject to magnetizing inrush currents which will appear to be an internal fault. Magnetizing inrush currents can occur when the transformer is initially energized, when the transformer recovers from an external fault, or when a nearby transformer is energized. The magnetizing inrush current has a large second harmonic component. To prevent operation of the relay due to magnetizing inrush current, a harmonic restraint unit is used to detect the magnetizing inrush current. The harmonic restraint unit detects the magnetizing inrush current by comparing the differential current with the second harmonic or 120 Hz component of the differential current. The harmonic restraint unit prevents operation of the relay when the second harmonic component exceeds a predetermined percentage of the differential current.
Overexcitation of the protected transformer can generate an apparent differential current to flow which is not attributable to an internal fault. Although potentially dangerous, overexcitation is not an internal fault and therefore the relay should not operate. The overexcitation current has a complex waveform consisting principally of third and fifth harmonics. To prevent operation of the relay due to overexcitation, a harmonic restraint unit is used to detect the overexcitation current. The harmonic restraint unit detects the overexcitation current by comparing the differential current with the fifth harmonic component of the differential current. The harmonic restraint unit prevents operation of the relay when the fifth harmonic component exceeds a predetermined percentage of the differential current.